I had quite the same issue trying to do the exact same thing ...
I have 3 volumes and wanted to shrink that to only one volume ...
Turned out even if I had everything moved to another volume -- I essentially relocated everything from the apps/service configuration window (Drive, Moments, Notes, Homes ...) when they permit it (almost no manual folder moves) -- , and it allowed me to remove the unrequired volumes, if I deleted the volume where the Photos share/folder was initially created (/volumeX/Photos) then it would not allow to start Moments. After digging into the logs I noticed that it was trying to access the Photos share/folder where my Moments Shared Library was and it had not taken into account that it had moved (/volumeY/Photos) so it always tried to reach it at its former location (/volumeX/Photos)... I tried to re-index everything but nothing changed ...
Tried to uninstall/reinstall Drive and Moments and while it allowed me to uninstall, it did not allow me to reinstall Drive and since Moments can't work without Drive ...
So I decided to recreate the Volume with smaller size and only with the Photos share/folder, reinstalled Drive and Moments and it worked again...
So not sure exactly how all this works in Drive/Moments DB but messing with the initial locations may cause issues ... I left it like that for now ... a bit disapointed.
TL;DR : tried to move all my shares to only one volume; everything OK except Moments was not restarting; turned out it was still trying to reach one share on a volume I had deleted and even worse although I was able to uninstall both Drive and Moments I was not able to reinstall; recreated the volume and share but smaller and everything worked again ; conclusion : don't mess with locations used by Drive and Moments.